The present invention relates generally to micromachined structures. More specifically, the present invention relates to released or suspended multilayer microstructures having a thick layer of rigid dielectric material (e.g. silicon nitride) and a thin layer of conductive polysilicon.
Released microstructures are commonly used in a variety of sensors, actuators and other useful devices. Released microstructures are suspended above a substrate (e.g. silicon) to which they are usually attached. Examples of released microstructures include comb drives, cantilevers, electrostatic motors and a wide variety of sensors (e.g. pressure sensors, magnetic sensors).
Released microstructures are often made from polysilicon. This is because polysilicon can be conformally deposited on many surfaces and it can be doped to provide conductivity. Also, polysilicon is easily released because there are a number of supporting materials available that can be selectively etched from under a polysilicon layer (e.g. phosphosilicate glass, PSG). However, polysilicon has the great disadvantage that deposited polysilicon layers have relatively high internal stress. Therefore, polysilicon structures tend to distort and bend when released. The tendency of polysilicon to bend after release is undesirable for making precision micromachined structures.
Polysilicon can be annealed to reduce internal stress and reduce bending. However, polysilicon annealing techniques are cumbersome and can interfere with other process steps required in making a useful device. It would be an improvement in the art to obviate polysilicon annealing in making certain released microstructures.
Another disadvantage of polysilicon is that it can have a relatively low strength.
Due to these disadvantages of polysilicon, silicon nitride is sometimes used instead for released microstructures. Low stress silicon nitride films are readily formed without annealing. Also, silicon-rich silicon nitride is rigid, strong, and can be released from a variety of supporting layers. A problem with silicon nitride is that it is an electrical insulator. Devices that require a conductive released microstructure cannot be made from silicon nitride.
Therefore, there is a need in the art for an electrically conductive material that can form low internal stress, high strength microstructures. Such a material could be used in a wide variety of released microstructures.
U.S. Pat. No. 5,936,159 to Kano et al. Discloses a released cantilever having a three-layer structure. The middle layer is a very thin stress relieving layer that tends to equalize stress in the cantilever, thereby reducing bending. In a preferred embodiment, a stress relieving layer tens of angstroms thick is disposed between thicker films of polysilicon.
U.S. Pat. No. 5,475,318 to Marcus et al. discloses a micromachined cantilever probe for contacting integrated circuits. The cantilever has two layers with different coefficients of expansion. When heated, the cantilever bends to provide electrical contact with a nearby electrical pad.
U.S. Pat. No. 5,866,805 to Han et al. discloses a cantilever having a magnetic thin film. The magnetic thin film provides magnetic coupling to a nearby electromagnet. The electromagnet can cause the cantilever to vibrate for use in xe2x80x98AC modexe2x80x99 force microscopy. A second layer is applied to the cantilever to reduce bending of the cantilever.
U.S. Pat. No. 5,796,152 to Carr et al. Discloses a cantilever having to separately bendable actuator sections, Each section can be heated separately. In this way, the cantilever can be caused to bend in complex shapes such as S-curves.
Accordingly, it is a primary object of the present invention to provide a released microstructure that:
1) has a very low internal stress and experiences minimal bending or distortion when released;
2) is electrically conductive;
3) can be made from materials easily applied using LPCVD or PECVD processes;
4) does not require annealing to achieve low internal stress;
5) can rapidly dissipate electrostatic charge.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by a microstructure having a silicon nitride layer and a conductive polysilicon layer attached to the silicon nitride layer. The polysilicon layer has a thickness less than ⅕ the thickness of the silicon nitride layer. The mechanical properties of the microstructure are primarily determined by the silicon nitride because the polysilicon is so thin.
More preferably, the conductive polysilicon layer thickness is less than {fraction (1/10)}, {fraction (1/20)}, {fraction (1/30)}, {fraction (1/50)}, or {fraction (1/100)} the silicon nitride layer thickness.
The silicon nitride layer and polysilicon layer can have a range of thicknesses. Preferably, the silicon nitride layer thickness is in the range of about 2-50 microns, and the polysilicon layer thickness is in the range of 5-50 nanometers.
Preferably, the polysilicon layer has a dopant concentration of at least 1017 atoms per cubic centimeter. More preferably, the polysilicon has a dopant concentration of at least 1018 or 1020 atoms per cubic centimeter.
Also preferably, the silicon nitride and polysilicon layers are LPCVD deposited layers. Preferably they are deposited in the same LPCVD step. The silicon nitride and polysilicon layers can also be deposited using plasma-enhanced chemical vapor deposition (PECVD).
Also, the polysilicon layer can be disposed on top of the silicon nitride layer or on the bottom of the silicon nitride layer. Also, the polysilicon layer can be disposed within the silicon nitride layer.
Also, the polysilicon layer can cover sidewalls of the silicon nitride layer. Also, the silicon nitride layer can be enclosed by the polysilicon layer. Also, multiple polysilicon layers can be disposed within the silicon nitride layer.